Optimize `Unit` returns

[krzema12]

Originally created by @SerVB.

If it's just Unit, just return Python's None; if it's nullable (Unit?), then do some more complex logic like null -> None and Unit -> Unit.getInstance(). However doing this will also require being careful with Unit assignments, for example, if there is code like val b = println(), it should be compiled like println(); val b = Unit (achievable via introducing an extra lowering) because println() will return None and it's not what we want to assign to b.

[SerVB]

Related fix on the JS side: https://youtrack.jetbrains.com/issue/KT-51127#focus=Comments-27-5940910.0-0 / https://github.com/jetbrains/kotlin/commit/51bb548aefc5a536dfaa540357964eb4fbd0391c (remove Unit_getInstance calls in statements)